Easy, no question about it, the 100-400. He WILL need the versatility of the zoom. Especially with big animals like elephants, 300 will often be too long and there will be times when the extra 100mm on the long end will also be useful.

+1

Keith Z Leonard
wrote:

Apparently later model 100-400s are better wide open, or at least that's the word on the street, I agree with you that mine is very good wide open and I only stop down when I need to for DOF. >

+1

You can't beat the 100-400 for flexibility on safari. As far as I know Canon sorted out the IQ problems about 4 years ago. Newer lenses are very, very good wide open.